In UMTS (Universal Mobile Telecommunications System) networks, for the purpose of improving spectral efficiency, peak data rates, etc., by adopting HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), it is performed exploiting maximum features of the system based on W-CDMA (Wideband Code Division Multiple Access). For the UMTS network, for the purpose of further increasing spectral efficiency and peak data rates, reducing delay and the like, Long Term Evolution (LTE) has been studied (Non-patent Document 1). In LTE, as distinct from W-CDMA, as a multiple access scheme, the scheme based on OFDMA (Orthogonal Frequency Division Multiple Access) is used in downlink, and the scheme based on SC-FDMA (Single Carrier Frequency Division Multiple Access) is used in uplink.
As shown in FIG. 1, signals transmitted in uplink are mapped to appropriate radio resources, and are transmitted from a mobile terminal apparatus to a radio base station apparatus. In this case, user data (UE (User Equipment) #1, UE #2) is assigned to the uplink shared channel (PUSCH: Physical Uplink Shared Channel), and control information is time-multiplexed with a data signal in the PUSCH when the control information is transmitted concurrently with the user data, while being assigned to the uplink control channel (PUCCH: Physical Uplink Control Channel) when only the control information is transmitted. The control information transmitted in uplink includes downlink quality information (CQI: Channel Quality Indicator, PMI: Precoding Matrix Indicator), retransmission response (ACK: Acknowledgement/NACK: Negative ACK) to the downlink shared channel, etc.
In the PUCCH, typically, different subframe structures are adopted between the case of transmitting CQI/PMI and the case of transmitting ACK/NACK (FIGS. 2(a), 2(b)). The subframe structure of the PUCCH is comprised of two slots, and one slot (½ subframe) contains 7 SC-FDMA symbols. Further, one SC-FDMA symbol contains 12 information symbols (subcarriers). More specifically, as shown in FIG. 2(a), in the subframe structure (CQI/PMI format) of CQI/PMI, a reference signal (RS) is multiplexed into a second symbol (#2) and sixth symbol (#6), and the control information (CQI/PMI) is multiplexed into the other symbols (first symbol, third to fifth symbols, seventh symbol) in a slot. Meanwhile, as shown in FIG. 2(b), in the subframe structure (ACK/NACK format) of ACK/NACK, a reference signal is multiplexed into third symbol (#3) to fifth sixth symbol (#5), and the control information (ACK/NACK) is multiplexed into the other symbols (first symbol (#1), second symbol (#2), sixth symbol (#6), seventh symbol (#7)) in a slot. In one subframe, the slot is repeated twice. Further, as shown in FIG. 1, the PUCCH is multiplexed into radio resources at opposite edges of the system band, and frequency hopping (Inter-slot FH) is applied between two slots having different frequency bands in one subframe.
When uplink control information of a plurality of users is multiplexed onto the PUCCH, the uplink control channel signals are orthogonally multiplexed so that the radio base station apparatus is capable of demultiplexing the uplink control channel signals for each user. As such an orthogonally multiplexing method, for example, there is an orthogonally multiplexing method using the cyclic shift of CAZAC (Constant Amplitude Zero Auto Correlation) code sequences.
The orthogonally multiplexing method using the cyclic shift of CAZAC code sequences is an orthogonally multiplexing method employing the fact that a sequence obtained by cyclically shifting a CAZAC code sequence with code length L by Δp is mutually orthogonal to a sequence obtained by cyclically shifting the CAZAC code sequence with code length L by Δq. In this method, for example as shown in FIG. 3, for UE #p, a CAZAC code sequence with code length L is cyclically shifted by Δp, and for UE #q, the same CAZAC code sequence with code length L is cyclically shifted by Δq. Further, since signals are transmitted by modulating (block modulation) the entire single SC-FDMA symbol with control information, orthogonality of uplink control channel signals is maintained among users. In addition, the interval of the cyclic shift of the CAZAC code sequence assigned to the user is preferably set to be longer than the maximum delay amount of multipath.
Herein, the orthogonally multiplexing method using the cyclic shift will be specifically described.
FIG. 4(a) is a diagram illustrating the cyclic shift of the same CAZAC code sequence (transmission signal sequence) in the time domain, and FIG. 4(b) is a diagram illustrating the cyclic shift of the same CAZAC code sequence in the frequency domain. FIGS. 4(a) and 4(b) are in a uniquely corresponding relationship (Fourier transform pair).
FIG. 4(a) illustrates 12 information symbols a0 to a11 in a single SC-OFDM symbol in the time domain. Between UE #p and UE #q, the cyclic shift is given by two information symbols (sequences are shifted by positions of white arrows in FIG. 4(a).) Meanwhile, FIG. 4(b) illustrates 12 subcarriers A0 to A11 (transmission band) in the frequency domain. In FIG. 4(b), in the UE #p and UE #q, each subcarrier is provided with different phase rotation associated with the cyclic shift shown in FIG. 4(a) (the direction of the arrow in FIG. 4(b) shows the rotation direction of the phase.) Therefore, the radio base station apparatus receives an addition of signals of the UE #p and UE #q. Accordingly, for example in FIG. 4(b), by making signals of 12 subcarriers of the UE #p coherent and adding and averaging over 12 subcarriers A0 to A11 signal components of the UE #q are canceled, and only signal components of the UE #p are left. By this means, it is possible to orthogonalize signals between users and demultiplex. In addition, on the condition that the amplitude (power) of a signal of each subcarrier is the same, complete orthogonality between users is achieved.